In indoor decoration, ceiling is an important part used to cover the rough, ugly concrete floor structure of the upstairs. One way to secure the ceiling under a concrete floor structure is to form a ceiling frame in a lattice form with elongated bars. Ceiling plates are then attached on the frame to complete the construction of the ceiling. For general houses or general purpose buildings, the ceiling frame is usually constituted by wooden bars for being easy to handle. Nails or screws are required to secure the wooden bars together to form the lattice frame and to fix the frame under the concrete floor structure.
Since nails or screws are used to fix the bars together, the strength of the ceiling frame is definitely dependent upon the use and number of the nails or screws. However, for a large frame, a carpenter may not be able to control the final quality of each joint of the frame. Namely, the nailing and structural strength may be variable from a joint to another. This leads in defects or faults in the overall strength of the frame for supporting the ceiling plates and other ornaments thereon and as a consequence, under a great load, the ceiling may crack and rend and even falling down.
It is therefore desirable to provide a standardized frame joint which provides a unified joint quality for securing bar members together to form the frame structure so as to avoid local defects or faults which may eventually lead in an overall crush of the frame.